PennHIP Evaluation

PennHIP differs from other methods in some very fundamental and important ways:

Scientific Protocol

PennHIP was developed and tested following strict scientific protocol and the results of these studies have been, and continue to be, published in peer-reviewed scientific journals. More than a decade of research and analysis have produced a body of information in support of PennHIP's effectiveness. (As with all diagnostic tests, PennHIP's accuracy is not 100%, but in direct comparisons it is far superior to any other available diagnostic methods.) No other method, published or practiced, has similar compelling scientific support.

Objective Assessment

PennHIP's evaluation protocol quantitatively measures passive joint laxity. Based on the degree of laxity, the individual dog is ranked relative to other members of the same breed. This allows breeders to easily identify animals with tighter hips within each breed. Dogs with tighter hips are less likely to develop osteoarthritis (hip dysplasia). Note that the hip evaluation report is not issued in a subjective pass/fail framework.This approach was adopted because some breeds of dogs have few members with hips tight enough to be considered truly DJD non-susceptible. In such breeds, genetic progress can be made (while still maintaining adequate genetic diversity) simply by breeding dogs in the tighter half of the population. Of course, greater selection pressure equates to more rapid genetic change.

Patient Positioning

Because PennHIP is measuring maximal passive hip laxity, the position of the patient is very different from the hip-extended position. The legs are in a neutral or stance-phase position rather than the conventional hip-extended position. The hip-extended position has been used for more than 37 years to screen hips for either DJD, laxity or both. However, studies have indicated wide diagnostic variability among radiologists in interpreting this view. Further, through biomechanical testing, the hip-extended view was found to mask the underlying true joint laxity and through direct comparison, the predictive value for CHD was shown to be inferior to the PennHIP procedure.

Heritability

Heritability is an important statistic relating the variation of a trait attributable to additive genetic effects with the total phenotypic variation of the trait. In other words, heritability relates the genetic makeup of a disease or trait with what is actually expressed or observed outwardly. Heritability is expressed as a number between 0 and 1. The higher the heritability, the greater the rate of genetic change that can be derived from selection pressure. The accuracy of a diagnostic test to determine disease (in this case CHD), has profound impact on the value of the heritability statistic. Inaccuracy of a diagnostic test acts to lower the estimate of heritability.

The heritability of the diseased phenotype evaluated in the hip-extended view has not been studied in most breeds of dogs in the USA. PennHIP is working with many breed clubs with an interest in determining the heritability estimates for their particular breed. Estimates for the heritability of passive hip laxity (DI) drawn from analysis of full pedigrees for the breeds examined thus far have yielded high values (e.g. for German Shepherd Dogs, heritability = 0.48; Labrador retrievers, heritability = 0.60).

Estimates of heritability of the dysplastic phenotype subjectively scored from the conventional hip-extended radiograph are not available for most breeds of dogs in the USA. Only a few published reports in as many breeds of dogs exist worldwide. To our knowledge, the OFA has not published heritability estimates for the subjective OFA phenotype.

Quality Control

The PennHIP method is based on strict quality control. To take PennHIP radiographs, veterinarians must undergo a training and certification process to demonstrate competency. PennHIP films are critically evaluated and the veterinarians are asked to repeat the procedure if the films do not meet PennHIP's stringent standards. The data generated from PennHIP undergoes regular review and statistical analysis so that useful information, by breed, is available to judge progress toward reducing CHD.

Mandatory Film Submission

For optimal validity, it is mandatory that all hip radiographs by PennHIP veterinarians be submitted for analysis and inclusion in the PennHIP database. This policy eliminates the practice of "prescreening" radiographs and sending only the best hips for evaluation. This "prescreening" practice has been shown to result in a biased database containing a greater frequency of non-diseased hips than actually exists in the true population. Excluding the worst hips leads to a false impression that genetic progress is being made.